I was 7 at the time and I remember going on the Friday afternoon of the week when it opened (it opened on a Wednesday). I went with my Mom and I remember thinking that Vader was lying to Luke. So for three years I always thought it was a trick from Vader and that Luke was too quick to believe him. Even watching ROTJ for the first time in '83 (saw that opening night), I still thought, "when is Luke going to realize it's a trick?!?" And of course I was distraught by Han's unknown fate. I was fully expecting another half hour of them going to rescue Han. I couldn't believe they would end the movie that way!
I also remember that someone in my school bus had a few of the ESB action figures well before the movie came out. I remember vividly he had Hoth Han and it was a good few months before the movie! I remember being so jealous, but he was in like 4th or 5th grade and I was only in 1st grade so I was very intimidated by him (he was a bit of a trouble maker) and didn't want to ask where he got the figures!
I even remember the teaser they showed of ESB when they did a re-release of SW in 1979. At the time it was a complete surprise that there was going to be a new SW film!